1) A devious ploy, which involves trickery and the element of surprise. (The most literal definition is inserting your P into a girls A instead of her V)
2) Any situation with your friends where you come from seemingly nowhere to surprise them
*similar concept to a back door cut in basketball... but with more real world application

jeez, i was gonna hook up with cait, had that shit all set up, and then u run the back door sneak on me?!?" "sorry bro, im sure she'll still fuck you too